Async replication can be consistent if it preserves transaction ordering and boundaries. In MongoDB that boundary is around each one update/insert operation. So

Async replication can be consistent if it preserves transaction ordering and boundaries. In MongoDB that boundary is around each one update/insert operation. So async replication isn't the consistency loss - the problem is that transaction boundaries are only weakly respected within one operation. The old data is not preserved while its new value is being written. Catastrophic shutdown could leave a record half-altered.